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  1. #include <iostream>
  2. #include <memory>
  3.  
  4. template<typename T>
  5. struct A {
  6.   T value_;
  7.  
  8.   A(T value) : value_(value) {}
  9.  
  10.   void sayHello() const {
  11.     std::cout << "Hello from A! " << value_ << '\n';
  12.   }
  13. };
  14. template<typename T>
  15. struct B {
  16.   T value_;
  17.  
  18.   B(T value) : value_(value) {}
  19.  
  20.   void sayHello() const {
  21.     std::cout << "Hello from B! " << value_ << '\n';
  22.   }
  23. };
  24.  
  25. class Wrapper {
  26. private:
  27.   class Concept {
  28.   public:
  29.     virtual ~Concept() = default;
  30.  
  31.     virtual void sayHello() const = 0;
  32.   };
  33.  
  34.   template<typename T>
  35.   class Model final
  36.       : public Concept {
  37.   private:
  38.     T data_;
  39.  
  40.   public:
  41.     Model(T data) : data_(data) {}
  42.  
  43.     virtual void sayHello() const override {
  44.       data_.sayHello();
  45.     }
  46.  
  47.   private:
  48.     template<typename U>
  49.     friend inline void doSomething(const Concept &lhs, const B<U> &rhs) {
  50.       T x = static_cast<const Model<T> &>(lhs).data_;
  51.  
  52.       x.sayHello();
  53.       rhs.sayHello();
  54.  
  55.       auto y = x.value_ + rhs.value_;
  56.  
  57.       std::cout << y << '\n';
  58.     }
  59.   };
  60.  
  61.   template<typename U>
  62.   friend inline void doSomething(const Concept &lhs, const B<U> &rhs);
  63.  
  64.   std::shared_ptr<const Concept> ptr_;
  65.  
  66. public:
  67.   template<typename T>
  68.   explicit inline Wrapper(T a)
  69.       : ptr_(std::make_shared<Model<A<T>>>(std::move(a))) {}
  70.  
  71.   template<typename U>
  72.   friend inline void someFriend(Wrapper &lhs, B<U> &rhs) {
  73.     doSomething(*lhs.ptr_, rhs);
  74.   }
  75. };
  76.  
  77. int main() {
  78.   Wrapper a(1);
  79.   B<int> b{2};
  80.  
  81.   someFriend(a, b);
  82.  
  83.   return 0;
  84. }
Success #stdin #stdout 0s 15240KB
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stdin
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Standard input is empty
stdout
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Hello from A! 1
Hello from B! 2
3
https://ideone.com/yqOBpH
language:
C++ (gcc 8.3)
created:
5 years ago
visibility:
public

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